1. Field of the Invention
The present invention relates to a tau-dither circuit used in a spread spectrum communications system.
2. Related Background Art
Demodulating a spread spectrum signal by using a tau-dither circuit has been carried out conventionally in the following manner as illustrated in the block diagram shown in FIG. 2. A received signal is multiplied at a multiplier 1 by a PN code which was phase-modulated by a phase modulator 10, to thereby despread the received signal. The output of the multiplier 1 is supplied to a demodulator 2 to demodulate it. The output of the multiplier 1 is also supplied via a band-pass filter 3 to a detector 4 which detects the envelope of the input signal. The envelope detected signal is multiplied at another multiplier 5 by a square wave reference signal outputted from a square wave reference signal oscillator 6, to thereby detect a phase difference between the received signal and PN code. The output of the multiplier 5, i.e., the detected phase difference error, is delivered to a loop filter 7 to obtain therefrom a dc output which is supplied to a voltage-controlled oscillator 8 as its control voltage. The output signal of the voltage-controlled oscillator 8 is supplied to a PN code generator 9 in the form of clock pulse. The PN code outputted from the PN code generator 9 is supplied to a delay circuit 11 and delayed by a predetermined amount. The delayed PN code from the delay circuit 11 and the PN code directly outputted from the PN code generator 9 and supplied to a switch 12. The switch 12 operates in response to the square wave reference signal from the square wave reference signal oscillator 6, to thereby phase-modulate the PN signal which is then supplied to the multiplier 1 for the despreading of the received signal. The delay circuit 11 and switch 12 constitute the phase modulator 10.
In the conventional tau-dither circuit described above, the same tau-dither circuit is used for both despreading of the received signal and synchronous tracking of the PN code to the received signal. Therefore, the conventional tau-dither circuit is associated with a contradictory problem that if the quality of the received signal after despreading is intended to be improved, the performance of synchronization retention of the PN code is degraded, or conversely if the performance of synchronization retention of the PN code is intended to be improved, the quality of the received signal after despreading is lowered.